Third generation mobile phone networks use CDMA (Code Division Multiple Access) spread spectrum signals for communicating across the radio interface between a mobile station and a base station. These 3G networks, (and also so-called 2.5G networks), are encompassed by the International Mobile Telecommunications IMT-2000 standard (www.ituint, hereby incorporated by reference). Third generation technology uses CDMA (Code Division Multiple Access) and the IMT-2000 standard contemplates three main modes of operation, W-CDMA (Wide band CDMA) direct spread FDD (Frequency Division Duplex) in Europe and Japan, CDMA-2000 multicarrier FDD for the USA, and TD-CDMA (Time Division Duplex CDMA) and TD-SCDMA (Time Division Synchronous CDMA) for China.
Collectively the radio access portion of a 3G network is referred to as UTRAN (Universal Terrestrial Radio Access Network) and a network comprising UTRAN access networks is known as a UMTS (Universal Mobile Telecommunications System) network. The UMTS system is the subject of standards produced by the Third Generation Partnership Project (3GPP, 3GPP2), technical specifications for which can be found at www.3gpp.org. These standards include Technical Specifications 23.101, which describes a general UMTS architecture, and 25.101 which describes user and radio transmission and reception (FDD) versions 4.0.0 and 3.2.2 respectively of which are hereby incorporated by reference.
FIG. 1 shows a generic structure of a third generation digital mobile phone system at 10. In FIG. 1 a radio mast 12 is coupled to a base station 14 which in turn is controlled by a base station controller 16. A mobile communications device 18 is shown in two-way communication with base station 14 across a radio or air interface 20, known as a Um interface in GSM (Global Systems for Mobile Communications) networks and GPRS (General Packet Radio Service) networks and a Uu interface in CDMA2000 and W-CDMA networks. Typically at any one time a plurality of mobile devices 18 are attached to a given base station, which includes a plurality of radio transceivers to serve these devices.
Base station controller 16 is coupled, together with a plurality of other base station controllers (not shown) to a mobile switching centre (MSC) 22. A plurality of such MSCs are in turn coupled to a gateway MSC (GMSC) 24 which connects the mobile phone network to the public switched telephone network (PSTN) 26. A home location register (HLR) 28 and a visitor location register (VLR) 30 manage call routing and roaming and other systems (not shown) manage authentication, billing. An operation and maintenance centre (OMC) 29 collects the statistics from network infrastructure elements such as base stations and switches to provide network operators with a high level view of the network's performance. The OMC can be used, for example, to determine how much of the available capacity of the network or parts of the network is being used at different times of day.
The above described network infrastructure essentially manages circuit switched voice connections between a mobile communications device 18 and other mobile devices and/or PSTN 26. So-called 2.5G networks such as GPRS, and 3G networks, add packet data services to the circuit switched voice services. In broad terms a packet control unit (PCU) 32 is added to the base station controller 16 and this is connected to a packet data network such as Internet 38 by means of a hierarchical series of switches. In a GSM-based network these comprise a serving GPRS support node (SGSN) 34 and a gateway GPRS support node (GGSM) 36. It will be appreciated that both in the system of FIG. 1 and in the system described later the functionalities of elements within the network may reside on a single physical node or on separate physical nodes of the system.
Communications between the mobile device 18 and the network infrastructure generally include both data and control signals. The data may comprise digitally encoded voice data or a data modem may be employed to transparently communicate data to and from the mobile device. In a GSM-type network text and other low-bandwidth data may also be sent using the GSM Short Message Service (SMS).
In a 2.5G or 3G network mobile device 18 may provide more than a simple voice connection to another phone. For example mobile device 18 may additionally or alternatively provide access to video and/or multimedia data services, web browsing, e-mail and other data services. Logically mobile device 18 may be considered to comprise a mobile terminal (incorporating a subscriber identity module (SIM) card) with a serial connection to terminal equipment such as a data processor or personal computer. Generally once the mobile device has attached to the network it is “always on” and user data can be transferred transparently between the device and an external data network, for example by means of standard AT commands at the mobile terminal-terminal equipment interface. Where a conventional mobile phone is employed for mobile device 18 a terminal adapter, such as a GSM data card, may be needed.
In a CDMA spread spectrum communication system a baseband signal is spread by mixing it with a pseudorandom spreading sequence of a much higher bit rate (referred to as the chip rate) before modulating the rf carrier. At the receiver the baseband signal is recovered by feeding the received signal and the pseudorandom spreading sequence into a correlator and allowing one to slip past the other until a lock is obtained. Once code lock has been obtained, it is maintained by means of a code tracking loop such as an early-late tracking loop which detects when the input signal is early or late with respect to the spreading sequence and compensates for the change.
Such a system is described as code division multiplexed as the baseband signal can only be recovered if the initial pseudorandom spreading sequence is known. A spread spectrum communication system allows many transmitters with different spreading sequences all to use the same part of the rf spectrum, a receiver “tuning” to the desired signal by selecting the appropriate spreading sequence.
In a 3G mobile phone system the base band data is spread using a spreading or channelisation code using an Orthogonal Variable Spreading Factor (OVSF) technique. The OVSF codes allow the spreading factor to be changed whilst maintaining orthogonality between codes of different lengths. To increase the number of simultaneous users of the system the data is further spread by a scrambling code such as a Gold code. The scrambling code does not change the signal bandwidth but allows signals to or from different users to be distinguished from one another, again, because the spreading codes are substantially mutually orthogonal. The scrambling is used on top of the channelisation spreading, that is a signal at the chip rate following OVSF spreading is multiplied by the scrambling code to produce a scrambled code at the same chip rate. The chip rate is thus determined by the channelisation code and, in this system, is unaffected by the subsequent scrambling. Thus the symbol rate for a given chip rate is likewise unaffected by the scrambling.
In a 3G mobile phone system different spreading factors and scrambling code links are generally employed for the down link from the base station to the mobile station and for the up link from the mobile station to the base station. Typically the channelisation codes have a length of between 4 chips and 256 chips or, equivalently, a spreading factor of between 4 and 256 (although other spreading factors may be employed). The up link and down link radio frames generally last 10 ms, corresponding to a scrambling code length of 38400 chips although shorter frames, for example of 256 chips, are sometimes employed on the up link. A typical chip rate is 3.84 M chips/sec (Mcps), which determines the maximum bit rate for a channel—for example with a spreading factor of 16, that is 16 chips per symbol, this gives a data rate of 240 Kbps. It will be recognised that the foregoing figures are provided merely for the purposes of illustration. Where higher bit rate communications with a mobile station are required more than one such channel may be employed to create a so-called multicode transmission. In a multicode transmission a plurality of data channels are used, effectively in parallel, to increase the overall rate of data transmission to or from a mobile station. Generally the multicode data channels have the same scrambling code but different channelisation codes, albeit preferably with the same spreading factor.
In a 3G mobile phone system there are generally a number of different channels some dedicated to particular users and some common to groups of users such as all the users within a given cell or sector. Traffic is carried on a Dedicated Physical Control Channel (DPCH), or on a plurality of such channels in the case of a multicode transmission, as described above. The common channels generally transport signalling and control information and may also be utilised for the physical layer of the system's radio link. Thus a Common Pilot Channel (CPICH) is provided comprising an unmodulated code channel scrambled with a cell-specific scrambling code to allow channel estimation and equalisation at the mobile station receiver. Similarly a Sychnronisation Channel (SCH) is provided for use by the mobile station to locate network cells. A primary SCH channel is unmodulated and is transmitted using the same channelisation spreading sequence in each cell and does not employ a cell-specific scrambling code. A similar secondary SCH channel is also provided, but with a limited number of spreading sequences. Primary and Secondary Common Control Physical Channel (PCCPCH, SCCPCH) having known channelisation and spreading codes are also provided to carry control information. The foregoing signalling channels (CPICH, SCH and CCPCH) must generally be decoded by all the mobile stations and thus the spreading codes (channelisation codes and where appropriate, scrambling code) will generally be known by the mobile station, for example because the known codes for a network have been stored in the user-end equipment. Here the references to channels are generally references to physical channels and one or more network transport channels may be mapped to such a physical channel. In the context of 3G mobile phone networks the mobile station or mobile device is often referred to as a terminal and in this specification no distinction is drawn between these general terms.
One advantage of spread spectrum systems is that they are relatively insensitive to multipath fading. Multipath fading arises when a signal from a transmitter to a receiver takes two or more different paths and hence two or more versions of the signals arrive at the receiver at different times and interfere with one another. This typically produces a comb-like frequency response and, when a wide band signal is received over a multipath channel, the multiple delays give the multiple components of the received signal the appearance of tines of a rake. The number and position of multipath channels generally changes over time, particularly when the transmitter or receiver is moving. However, as the skilled person will understand, a correlator in a spread spectrum receiver will tend to lock onto one of the multipath components, normally the direct signal which is the strongest.
As is known in the art a plurality of correlators may be provided to allow the spread spectrum receiver to lock onto a corresponding plurality of separate multipath components of the received signal. Such a spread spectrum receiver is known as a rake receiver and the elements of the receiver comprising the correlators are often referred to as “fingers” of the rake receiver. The separate outputs from each finger of the rake receiver are combined to provide an improved signal to noise ratio (or bit error rate) generally either by weighting each output equally or by estimating weights which maximise the signal to noise ratio of the combined output. This latter technique is known as Maximal Ratio Combining (MRC).
There is a general need to provide user-end terminal capable of supporting the higher data rates possible in 3G systems, particularly in areas with large numbers of users. It is generally thought that a CDMA system is uplink-limited due to the near-far effect (where the correlation with a strong, nearby signal having an incorrect code is greater than that with a weaker, more distant signal with the correct code).
To facilitate the support of higher data rate services it is known to employ Multiple Access Interference (MAI) suppression at the base station to improve the uplink. Multiple access interference rises because the spreading codes of signals received from different users are not normally completely orthogonal. Interference cancellation (IC) receivers in the base station thus attempt to estimate a multiple access interference component which is subtracted from the received signal, either in parallel across all the users or sequentially. The multiple access interference which is cancelled is the interference between the same multipath component of two substantially orthogonal received signals. This technique is described in more detail in Section 11.5.2 of “WCDMA for UMTS by H Holma and A Toskala, John Wiley & Sons, 2001” (ISBN0 741 48687 6).
A technique for suppressing interference between different multipath components of a single data channel, that is for suppressing Interpath Self-interference (IPI), has also been described in a paper by NTT Docomo, “Multipath Interference Canceller (MPIC) for HSDPA and Effect of 64QAM Data Modulation” (TSG RAN WG)1 Meeting #18, document (01) 0102 available from the 3GPP website at http://www.3gpp.org/ftp/tsg_ran/wg1_rl1/tsgr1—18/docs/pdfs/r1-01-0102.pdf).
Intracell interference arises due to interpath interference and the loss of orthogonality between the channelisation codes. In an ideal environment with a single path between the transmitter and the receiver the OVSF channelisation codes ensure that the different transmitted streams are (substantially) orthogonal to one another. However in the presence of multipath time dispersion the non-zero auto (or cross) space-correlation between different multipath components gives rise to interpath interference.
Consider the case where a spread spectrum receiver receives two signals simultaneously, a first signal with a first spreading code of 1-11-1 and a second signal with a second spreading code of 11-1-1. These two spreading codes are substantially orthogonal over a symbol period as they sum to −1. However if the second code is offset slightly with respect to the first code the non-orthogonal component increases. Such an offset can be caused by multipath which effectively introduces a delayed component of both the first and second signals, albeit normally at a reduced power. Considering for example the first spreading code, a non-orthogonal contribution arises both from the delayed version of the first code, because of the non-ideal auto correlation properties of the codes, and also from the delayed version of the second code, because of the non-ideal cross-correlation properties of the codes.
The relatively poor correlation properties of OVSF codes when not time-aligned are known and this is the reason why an additional spreading code is applied in W-CDMA 3G systems. As explained above, the codes used in W-CDMA, as specified by the 3GPP, are Gold codes formed from positionwise modulo to summation of 38,400 chip segments of two binary m-sequences. The auto correlation-properties of an m sequence are illustrated in FIG. 3 in which the correlation function is shown on y axis 300. With a non-zero offset the maximum correlation output is proportional to the reciprocal of the spreading length, at −1/S where s is the spreading length. The spreading length is itself determined by the member of elements, n, in the shift register used to generate the code. The delay offset, T, between subsequent auto correlation peaks is given by the code length, S, multiplied by the chip period, tc. With a large spreading factor 1/S will tend to zero and thus this code will approach the ideal characteristics of a zero auto correlation when not time-aligned. However at low spreading factors, which correspond to higher data rates, Interpath Interference (IPI) can become significant.
The capacity of CDMA systems is self-interference limited—that is the performance in terms of both capacity and quality of service, is determined to a large extent by the interference power arising from users within the same cell or in adjacent cells. It is therefore possible to improve the performance of CDMA systems by reducing this level of interference and there are a number of well known and accepted techniques for accomplishing this, including discontinuous transmission and the use of sectorised antennas. Interference within a cell can be mitigated to some degree based upon the recognition that signals from the base station to a terminal are synchronised and thus intra-cell MAI (Multiple Access Interference) can be mitigated by using codes which are orthogonal when aligned to within a chip period, such as the OVSF codes described above, or the Walsh codes used, for example, in IS95 (Interim Standard 95) CDMA phone networks in the USA. However in practice the time-dispersive nature of the mobile environment causes a significant loss of orthogonality, as described above, and a consequent increase in MAI. For example in a typical urban environment a loss of orthogonality of up to 40 percent may be observed. Inter-cell multiple access interference may also be observed.
The interference can be even greater on the uplink because the various mobile terminals communicating with the base station may not be time aligned, exacerbating the problem of multiple access interference. It is possible to improve the quality of service or capacity by reducing the level of this interference. There are a number of well-known and widely accepted techniques that reduce the level of this interference, such as discontinuous transmission and sectorised antennas.
As discussed, spreading codes that are orthogonal when aligned to a chip period (e.g. Walsh or OVSF codes) can be employed to minimise intra-cell multiple access interference (MAI). However, the codes ideal orthogonal properties only exist in a channel that exhibits no time dispersion and where the users are synchronised. In reality the time dispersive nature of the mobile environment and non-synchronous multi-user reception (particularly in the uplink configuration) can cause a significant loss of orthogonality and increase in MAI. In addition, inter-cell multiple access interference will also be observed.
On the uplink, as the base station (BS) will be receiving the desired and other users signals (non-synchronously), there will be an enormous level of interference from the other users. Unlike the downlink (DL) configuration, in the uplink the BS has the leverage of having a priori knowledge of all the users spreading sequences, thereby making it possible to remove (or suppress) the interference which the interfering users cause.
Such interference cancellation (IC) is described for example in US2002/0021682, and involves filtering each channel using its code (de-spreading) in order to determine its associated (or de-spread) symbol. The symbol is then re-coded (re-spread) in order to subtract this from the composite received signal, thus eliminating interference associated with that channel from the composite signal. The document further discloses a power control method which together with the interference cancellers can effectively increase the system capacity and make it resistant to sudden changes on the communication path. Also a power control method with a fast response is capable of preventing unnecessary increases in the transmission power (and multiple access interference) of the uplink by reflecting the values of the post-interference cancellation signal-to-interference power ratio in the generation of power control command information. A multiple access interference signal contained in a reception signal from the mobile station is cancelled, a post-interference cancellation signal-to-interference power ratio of the reception signal currently received is estimated, a power control command is generated by comparing the estimated post-interference cancellation signal-to-interference power ratio and a target value for power control, and transmitting this power control command to the mobile station to control the transmission power of the mobile station
K. Rizvi, M. Fitton, Y. Chow, M. Ismail and Y. Bian, “Common Channel Interference Cancellation for a Spread Spectrum system”, IEEE VTC 2004 Spring, May 2004 investigates advanced signal processing algorithms for existing and future wireless systems. This paper is focussed on evaluating the potential performance improvements that accrue from cancelling the common pilot channel's interference in terms of bit error rate performance for downlink applications. The impact of different cancellation strategies—sequential (SIC) and parallel (PIC)—is presented for various data rates and channel profiles. With the investigative analysis appertaining to the FDD element of the UTRAN specified by 3GPP, the performance of the interference cancellation unit is primarily based on the elimination of the cross-correlation of the spreading codes over time dispersive wireless conditions. With a sound channel estimate, the results are shown to achieve near interference free performance highlighting the potentiality of these interference cancellation schemes. Their versatility and robustness against channel estimation errors is further demonstrated by a relative performance comparison for a diversified range of data rates, pilot-to-data power ratios and channel conditions.
An alternative approach for the uplink attempts to improve performance by time aligning the mobile terminals T. Kawamura, H. Atarashi, and M. Sawahashi, “Adaptive Transmission Timing Control Using Reservation Packet in Reverse Link for DS-CDMA Broadband Wireless Access”, Technical report of IEICE, RCS2003-141 (2003-10 proposes the Adaptive Transmission Timing Control (ATTC) describes a method of utilizing reservation packets for DS-CDMA in the uplink, in order to increase the link capacity while maintaining advantages such as one-cell frequency reuse in a cellular system in a multi-cell configuration. This method is illustrated in FIG. 2. The difference in the received reservation packet timing due to the propagation delay time is measured by utilizing the reservation packets that contain small amounts of information such as the data types, i.e., real time or non-realtime, and the size of the traffic bits in the packet data channel. The transmission timings of the successive packet data channels are controlled so that the received timings of the highest signal power path of the packet data channels of simultaneously accessing mobile terminals are aligned.
EP1039662 describes uplink and downlink communications which are coded with a spread spectrum sequence, and wherein the mobile terminals have different powers. At the base station, interference cancellation (IC) is performed in order to reduce the interfering effect of the highest power signals on the detected lowest power signals. In this method, at least for some of the most powerful interfering signals, groups of symbols of different interfering signals are time aligned when they reach the central station. Preferably, the synchronized groups of symbols are synchronously detected at the base station and are processed jointly prior to be subtracted from the signal to be detected.
Whilst these various methods improve performance, there is still a need for further improvement, especially for broadband channels.